One of the most prominent annual rhythms expressed in animals is the photoperiod-dependent seasonal change in reproductive activity. While much progress has been made in understanding the physiological mechanisms underlying the photoperiodic control of reproduction at the system's level, very little is known about the cellular and molecular events within the central nervous system and the hypothalamic-pituitary axis that underlie the transition between the reproductively active and inactive states. The male Siberian hamster is an attractive model for such mechanistic studies for three reasons: 1) pronounced changes in reproductive neuroendocrine function can be induced within 1-2 days of photostimulation, 2) follicle stimulating hormone (FSH) is selectively synthesized and released following photostimulation, and 3) gonadotropin secretion can be induced in males housed in long photoperiod following exposure to a female, but this induction is inhibited in males exposed to short photoperiods. The overall objectives of the proposed project are to elucidate the early cellular and molecular events that underlie the photic induction of increased hypothalamic gonadotropin releasing hormone (GnRH) and pituitary FSH content and to determine how the photoperiod alters the processing of afferent signals in response to exposure to a female that ultimately influences pituitary gonadotropin release. A variety of experimental techniques (i.e., manipulation of hormone and neurotransmitter system by infusion of exogenous hormone or treatment with agonists or antagonists, detection of intracellular changes in mRNA and protein content using ribonuclease protection assays, quantitative rtPCR, in situ hybridization, immunocytochemistry, and radioimmunoassay) will be employed to achieve these objectives. The proposed studies are expected to yield new information about the fundamental mechanisms by which environmental information is integrated to regulate the activity of the hypothalamic-pituitary-gonadal axis and are expected to lead to an improved understanding of fertility and infertility. In view of the increasing evidence that both photoperiod and chemosensory signals can influence human reproduction, the proposed studies are also expected to lead to new insights into the fundamental mechanisms by which human reproduction can be influenced by environmental factors.